1. Field of the Invention
The present invention relates to a drop volume measurement and control mechanism and process for inkjet printing. More particularly, the present invention relates to the measurement of an electrical property of an ink-jet droplet, such as its dielectric properties, to determine its volume.
2. Description of Related Art
One conventional type of printer forms characters and images on a medium or substrate, such as paper, by expelling droplets of ink, often comprising organic material, in a controlled fashion so that the droplets land on the medium in a pattern. Such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet type printer comprises an ink containment device and a fingernail-sized apparatus, commonly known as a print head, which heats and expels ink droplets in a controlled fashion. The print cartridge may contain a storage vessel for ink, or the storage vessel may be separate from the print head. Other conventional inkjet type printers use piezo elements that can vary the ink chamber volume through use of the piezo-electric effect to expel ink droplets in a controlled fashion. Helpful background material may be found in U.S. patent application Ser. No. 10/191,911, entitled “Process And Tool With Energy Source For Fabrication Of Organic Electronic Devices”, which is incorporated herein by reference.
Ink jet printing is a relatively new technique for deposition of polymer solutions to create organic electronics (by way of example only, organic integrated circuit boards, thin film transistors, detectors, solar cells, displays based on light-emitting polymers). Other applications of ink jet printing include, by way of example only, ink-jet printing of color filter arrays such as OLEDs and LCD displays, printing of metal solutions/suspensions to create conductive/metal lines, and printing of materials for biomedical or bio-chemical applications and devices. In a typical application, polymers, monomers, and/or oligomers are dissolved or dispersed in appropriate solvents and are deposited onto appropriate substrates by an ink jet printing process. The solutions dry and form thin solid films on these substrates. For organic light-emitting devices (OLEDs), the thickness of these films is often measured in nanometers. Unintentional thickness variations and inhomogeneities may cause major defects in the end product. For example, in many circuit elements, current is roughly inversely proportional to the film thickness cubed. Thus, small thickness variations often cause unacceptable variations in current for the same driving voltage. Since the light output for OLEDs is approximately proportional to the current, variation in the thickness can create significant variation in the light output. If the film thickness needs to be within a certain range (such as a tolerance of ±5%), the volume of droplets ejected from ink jet nozzles has to be restricted to a similar tolerance.
Although drop volume must be carefully controlled for the creation of organic electronics using ink jet nozzles, drop volume is also an important consideration for other dispensing devices. By way of example only, ink jet printers for graphic arts or printers used for the creation of color filters for liquid crystal displays can also benefit from control of drop volume. Thus, dispensing devices for bio-chemistry and printing of polymeric integrated circuit boards are only some of the applications where drop volume is important. Piezo-based ink jet printing, thermal ink jet printing, microdosing, and micro-pipettes are just some of the types of dispensing devices that eject ink droplets.
“Off-line” methods exist to measure drop volume of ejected droplets. One method is to eject a defined number of droplets into a container and, using the weight of the resulting ejected droplets (or the resulting dried film/drop material) along with the known density, calculating the average drop volume. Helpful background material may be found in various publications, such as, by way of example only, S. F. Pond: “Inkjet Technology”, Torrey Pines Research (2000).
Disadvantageously, the off-line method, as the name implies, requires that the particular dispenser or dispensers being tested are taken out of use while being tested. The interruption of the printing process and the time consumption involved during testing can mean a significant decrease in productivity. Additionally, if there is more than one nozzle, each nozzle must be tested separately, and so it is not efficient to perform a determination of drop volume variation between nozzles. Furthermore, the evaporation of solvents in the droplets between the time the droplets leave the nozzle and the moment they are weighed can skew the results of the test.
Optical methods tend to be more sophisticated than the “off-line” method described above. Stroboscopic illumination of droplets may be used to take pictures of droplets during flight, and the drop diameter and drop volume are calculated from these images. Laser measurements can be used to determine the drop volume by measuring the length of time a laser beam is blocked by the droplet and, using that information along with the drop velocity measurements, calculating the drop diameter.
Disadvantageously, stroboscopic measurement is inaccurate. The visible border of a given droplet strongly depends on the illumination, camera settings, and other technical variations, making the results unreliable for many applications.
Laser measurements are generally more precise than stroboscopic measurements, but are also time-consuming and expensive. Furthermore, the optical components (such as mirrors, lenses, light-sources) used for laser measurements may be too bulky for a given application. The bulkiness of components is especially disadvantageous when attempting to implement a plurality of detectors that are capable of scanning a plurality of nozzles simultaneously. Additionally, the laser source may introduce laser hazards. Finally, liquid droplets having different components may have different absorption of light, thereby skewing the results.
Optical methods are also susceptible to being compromised by ink splashes and/or dirt in the environment. In the “dirty” environment of printing, the performance of optical sensors can be compromised, necessitating frequent cleaning and/or replacement of parts.